Selective Grain Boundary Engineering

ABSTRACT

A process for grain boundary engineering of an aluminum alloy of AA5XXX series which includes steps of annealing the aluminum alloy at a first temperature of from about 350° C. to about 450° C.; deforming the annealed aluminum alloy to reduce the thickness by from about 2% to about 20% of the original thickness of the aluminum alloy; heat treating the deformed aluminum alloy at a second temperature from about 450° C. to about 550° C., and optionally sensitizing the heat treated alloy in one or more sensitizing steps. Aluminum alloys of the AA5XXX series treated by the process of the present invention are also provided.

This invention was made with government support under Contract No. N000141210505 awarded by the Office of Naval Research. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the field of thermomechanical processing of aluminum alloy. In particular, the present invention is directed to a thermomechanical process for treating aluminum alloy to decrease the susceptibility of the aluminum alloy to intergranular corrosion.

2. Description of the Related Technology

Due to their light weight and good corrosion resistance properties as compared to steels, aluminum alloys have the potential of replacing steels in a wide range of applications, such as in aircraft, ships and boats, trucks, cars and other vehicles. For example, aluminum alloys of the AA5XXX series, which have magnesium as a major alloying element, have been broadly used in marine applications. AA5XXX series aluminum alloys may be sensitized to enhance resistance to corrosion, as a result of which magnesium atoms in Al—Mg alloys may precipitate out and form a β-phase (Al₃Mg₂) along the grain boundaries, especially in alloys with Mg levels above ˜3 wt. %.

The β-phase is anodic to the matrix material, leading to formation of small openings along the grain boundary network. These small openings within the matrix material may initiate a process called stress corrosion cracking (SCC). SCC is a type of cracking induced by the combined influence of tensile stress and a corrosive environment. SCC can lead to unexpected sudden failure of normally ductile metals when they are subjected to a tensile stress, due to the ability of SCC to cause gaps to grow faster within the aluminum alloy than otherwise expected.

To increase strength and/or corrosion resistance, aluminum alloys are traditionally cast into an ingot of approximately 12 to 28 inches in thickness. The ingot is then scalped and preheated, after which it may be hot rolled to about 0.125 inch, cold rolled to about 0.020 to 0.060 inch, and subjected to further heat treatment, such as batch annealing or solution heat treatment. One concern with such traditional approaches is the intermetallic particles present in the as-cast aluminum alloy ingots, which is a function of the alloy composition and the solidification rate in the casting process. The intermetallic particles can participate in a fracture initiation and propagation and, as a result, may limit the formability or design tolerance of the aluminum alloys. The intermetallic particles also act as void nucleation sites during sheet forming processes, such as stretching and, therefore, may contribute to fracture initiation within the aluminum alloy matrix.

Several thermomechanical processes have been developed attempting to improve various properties of aluminum alloys, especially their corrosion resistance. For example, U.S. Pat. No. 6,544,358 discloses several methods of processing aluminum alloy of the AA5XXX One method uses two steps of hot rolling and one step of cold rolling to produce a thin alloy sheet. The alloy sheet is then inter-annealed at a temperature of from 300 to 500° C. After annealing, the alloy sheet is cold rolled to reduce the thickness by 40-60%; and lastly, the cold rolled alloy is annealed at temperature from 300 to 500° C.

L. Tan & T. R. Allen, “Effect of thermomechanical treatment on the corrosion of AA5083,” Corrosion Science, vol. 52, pages 548-554 (2010) discloses a process of treating aluminum alloy AA5083-H116 to induce resistance to corrosion. The process includes the steps of cold working to reduce the thickness by about 25%, followed by annealing at 500° C. for 30 minutes and water quenching. The process is said to affect grain boundary characteristics, grain shape, texture, and precipitates formed by corrosion of the aluminum alloys.

U.S. Pat. No. 6,344,096 discloses a method for treating aluminum alloys including alloys of the AA5XXX series. The method involves the sequential steps of cold rolling the alloy to provide a sheet with a thickness of less than 0.15 inch starting from an alloy with thickness of 0.5 inch and continuous annealing at about 700 to 1100° F. for 1 to 60 seconds in order to cause one or more of the alloying constituents to enter solid solution. This is followed by sufficiently rapid cooling to minimize undesired constituent precipitation. Such cooling may be effected, for example, by forced air, water spray or water mist. The treated aluminum alloy sheet is said to have high strength and formability along with good surface quality.

U.S. Pat. No. 5,496,426 discloses a process said to improve the strength, toughness and corrosion resistance of an aluminum alloy. The process includes the successive steps of homogenizing, hot rolling and thermally treating or annealing at about 750° to 850° F. for 1 to 6 hours; followed by cold rolling to a reduction in thickness of between about 20 and 60%; followed by a two-stage thermal annealing treatment including heating, preferably within a temperature range of about 625 to about 725° F. for a period of about 1½ to 4 hours, followed by controlled cooling or ramping down to one or more temperatures within a range of 350 to 550° F. over a period of about 2 to 6 hours.

U.S. Pat. No. 6,350,329 discloses a process for treating age-hardened aluminum alloys, such as those of the AA2XXX, AA6XXX, AA7XXX and certain AA8XXX series. The process includes the successive steps of solution heat treating the alloy at about 540° C. for about one hour, rapidly cooling the alloy, plastically deforming the alloy by rolling at room temperature, under conditions sufficiently severe to produce a high-energy defect structure in the grains of the alloy, and aging the alloy to induce nucleation and growth of precipitates at a temperature of 380 to 450° C. for up to 24 hours. The process is preferably coupled with a multi-step low and high temperature aging process, and is designed to produce a uniform distribution of micron-size precipitates necessary for the subsequent development of a fine, equiaxed grain structure that is said to be stable at superplastic forming temperatures.

WO 2009/132436 discloses a thermomechanical process for treating an AA6XXX aluminum alloy to produce extended high temperature ductility. The process includes the sequential steps of solution heat treating the alloy followed by rapid cooling, plastically deforming the alloy by at least 30%, annealing by heating from a room temperature to high enough temperature (e.g. 300° C.) to induce re-crystallization and/or formation of precipitates, continuously heating the alloy at below the re-crystallization temperature for forming dispersed fine precipitates, and heating the alloy at or above the crystallization temperature to achieve a fine grain structure.

To improve corrosion resistance of the aluminum alloy in the AA5XXX series, the present invention employs a thermomechanical process to treat the aluminum alloy to selectively engineer the grain boundary. The engineered grain boundary impedes growth of β-phase in the alloy matrix. This process is capable of significantly reducing the aluminum alloy's susceptibility to sensitization, thus increasing its resistance to corrosion.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a process for grain boundary engineering of an aluminum alloy of the AA5XXX series. The process includes steps of annealing the aluminum alloy at a temperature of from about 350° C. to about 450° C.; deforming the annealed aluminum alloy to reduce the thickness by from about 2% to about 20% of the original thickness of the aluminum alloy; and heat treating the deformed aluminum alloy at a temperature of from about 450° C. to about 550° C.

In another aspect, the process of the present invention further includes the step of cooling the annealed aluminum alloy prior to the deforming step.

In yet another aspect, the process of the present invention further includes the step of sensitizing the heat treated aluminum alloy.

In yet another aspect, the process of the present invention includes a plurality of sensitizing steps that are each performed at a different temperature.

In yet another aspect, the present invention provides an aluminum alloy of the AA5XXX series treated by the process of the present invention.

These and various other advantages and features of novelty that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart representing a process for selective grain boundary engineering of an aluminum alloy of the AA5XXX series according to the present invention.

FIG. 2 is a plot of the mass loss analysis of samples processed in accordance with the method of Example 1.

FIG. 3 is a plot of the mass loss analysis for samples processed as in Examples 1 and 2 using a cold rolling thickness reduction in the range of 0%-20%.

FIG. 4A shows an electron backscatter diffraction image obtained by using a scanning electron microscope on an unprocessed aluminum alloy sample.

FIG. 4B shows an electron backscatter diffraction image obtained by using a scanning electron microscope on an aluminum alloy sample processed with a 5% cold rolling thickness reduction and 500° C. heat treatment as set forth in Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present disclosure are described by referencing various exemplary embodiments. Although certain embodiments are specifically described herein, one of ordinary skill in the art will readily recognize that the same principles are equally applicable to, and can be employed in other systems and methods. Before explaining the disclosed embodiments of the present disclosure in detail, it is to be understood that the disclosure is not limited in its application to the details of any particular embodiment shown. Additionally, the terminology used herein is for the purpose of description and not of limitation. Furthermore, although certain methods are described with reference to steps that are presented herein in a certain order, in many instances, these steps may be performed in any order as may be appreciated by one skilled in the art; the novel method is therefore not limited to the particular arrangement of steps disclosed herein.

It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. Furthermore, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. The terms “comprising”, “including”, “having” and “constructed from” can also be used interchangeably.

The present invention provides a process for selective grain boundary engineering of aluminum alloys for enhancing corrosion resistance. Aluminum alloys suitable for the process of the present invention include aluminum alloys of the AA5XXX series in the Aluminum Association Register. The AA5XXX series aluminum alloys have magnesium as the major alloying element.

The AA5XXX series of alloys are considered non-heat treatable because they cannot generally be appreciably strengthened by solution heat treatment. Instead, the AA5XXX series aluminum alloys are usually strengthened by solid-solution formation of second-phase microstructural constituents, dispersoid precipitates and/or strain hardening. In addition the AA5XXX series is readily weldable, and for these reasons these alloys may be used in a wide variety of applications such as shipbuilding, transportation, pressure vessels, bridges and buildings. These alloys are often welded with filler alloys, which are selected after consideration of the magnesium content of the base material, and the application and service conditions of the welded aluminum alloy.

Examples of suitable aluminum alloy include the aluminum alloy AA5083, for example, AA5083-H116. Aluminum alloy AA5083 is a commercially available aluminum alloy comprising about 4.4% Mg, 0.7% Mn, 0.15% Cr, with the balance being aluminum. This alloy is currently used in marine applications because it is considered to be resistant to seawater corrosion. The temper designation “H116” indicates a soft temper alloy that has been strain hardened without thermal treatment.

In one aspect, the present invention provides a process for selective grain boundary engineering of an aluminum alloy of the AA5XXX series. Referring to FIG. 1, the process comprises steps of annealing the aluminum alloy at a temperature of from about 350° C. to about 450° C.; deforming the annealed aluminum alloy to reduce the thickness of the aluminum alloy by from about 2% to about 20% of the original thickness of the aluminum alloy; and heat treating the deformed aluminum alloy at a temperature of from about 450° C. to about 550° C.

In some embodiments, the initial annealing step comprises solution heat treating the aluminum alloy at a temperature of from about 350° C. to about 450° C., or from about 370° C. to about 430° C., or from about 380° C. to about 420° C., or from about 390° C. to about 410° C. The time required for the annealing step should be sufficient to ensure that all magnesium in the aluminum alloy has gone into solution and that no previously formed β-phase from, for example, fabrication or shaping is present in the annealed aluminum alloy. A skilled person may ascertain the annealing time for a specific aluminum alloy. To assure that the magnesium is in solution one can maintain the temperature in the higher end of the temperature ranges given above, i.e. above about 400° C. Hardness testing can give some indication that the precipitates are gone as a result of the magnesium going back into solution. G67 corrosion testing can also be used to show that magnesium precipitates are gone as a result of the magnesium going back into solution.

In some embodiments, the annealing step is conducted for a period of from about 0.5 hour to about 3 hours, or from about 1 to about 2 hours.

In some embodiments, the annealing step may be performed in a solution heat treating furnace. The solution heat treating furnace must be capable of accurately controlling the furnace temperature and temperature variation must be limited to within a range of about ±5° C. Overheating should be avoided, especially not exceeding the initial eutectic melting temperatures of the aluminum alloy. Though not apparent at early stages of overheating, a deterioration of mechanical properties of the aluminum alloy may result from overheating the aluminum alloy.

After the annealing step, the aluminum alloy is cooled by, for example, air cooling or quenching. Although the mode of cooling is not critical, the annealed aluminum alloy is preferably cooled rapidly to a temperature at which the diffusion rate of the elements in the aluminum alloy matrix is not appreciable, and formation of precipitates, particularly on the grain boundaries of the aluminum alloy, is thereby prevented. In some embodiments, the cooling rate may be from about 10 to about 30° C. per minute, or from about 15 to about 25° C. per minute, or from about 18 to about 22° C. per minute, until the temperature of the aluminum alloy is reduced to below a desired temperature. This desired temperature may be from about 200° C. to about 300° C., or from about 230° C. to about 290° C., or from about 250° C. to about 290° C., in some embodiments. It is desirable to ensure that magnesium precipitation does not occur during cooling and thus ensuring a temperature below 200° C. Temperatures of 100° C. to 200° C. can usually be tolerated for a period of up to about 10 minutes while minimizing or prevent magnesium precipitation.

In one embodiment, the annealed aluminum alloy is quenched. Quenching can help to keep dissolved constituents in solution after cooling the annealed alloy to room temperature. The speed of quenching is important as excessive delay in transferring the aluminum alloys to a quenching medium may adversely affect the properties of the aluminum alloy. It is desirable to reduce the temperature to below 200° C. as soon as possible in order to minimize or prevent magnesium precipitation and thus quenching may be employed to achieve this goal. The quenching medium may be, for example, water or oil at room temperature. Other rapid cooling methods known to skilled persons may also be used for the present invention.

Referring to FIG. 1, the annealed aluminum alloy is then deformed to achieve a reduction in thickness of from about 2% to about 20%, or from about 3 to about 15%, or from about 5% to about 10% of the original thickness of the aluminum alloy. Any suitable method for deforming an alloy may be used, such as rolling, stretching, extrusion, pressing, drawing, forging, torsion processes, and any combination of these processes, among others, so long as the method is sufficiently severe to produce desired thickness reduction in the aluminum alloy. Preferably, the amount of reduction per pass and number of passes is such that the deformation fully penetrates, or substantially penetrates, the entire thickness of the aluminum alloy. It is also preferable that the deformation be uniform, or substantially uniform, throughout the thickness of the aluminum alloy.

In one exemplary embodiment, the deforming step comprises a cold rolling process, including both conventional cold rolling and asymmetric cold rolling. Cold rolling is a metal working process in which a metal is deformed by passing it through rollers at a temperature below its recrystallization temperature, such as at room temperature. Cold rolling increases the yield strength and hardness of the aluminum alloy by introducing defects into the alloy's crystal structure. These defects prevent further slip and can reduce the grain size of the aluminum alloy, resulting in improved hardness of the aluminum alloy. The cold rolling can be carried out in one or more roll bite passes. Any mode of cold rolling may be used, so long as it is sufficiently severe to produce the required thickness reduction. The advantages produced by cold rolling are dimensional accuracy and good surface finish.

Referring to FIG. 1, the deformed aluminum alloy is then heat treated at a temperature of from about 450° C. to about 550° C., or from about 475° C. to about 525° C., or from about 485° C. to about 515° C., or about 500° C. The time for this heat treatment step may be from about 1 to about 6 hours, or from about 2 to about 5 hours, or about 2 to about 4 heat treatment step may be carried out in a batch operation for economical efficiency. The cooling of the aluminum alloy after this heat treating step is not critical and the alloy may be slowly cooled to room temperature.

The heat treated alluminum alloy may optionally be subjected to a sensitizing step where the aluminum alloy is kept at a temperature of from about 100° C. to about 200° C., or about 120° C. to about 180° C., or about 140° C. to about 160° C. The sensitizing step may be performed for a period of from about 70 hours to about 150 hours, or from about 80 hours to about 130 hours, or from about 90 hours to about 120 hours. The times and temperatures of the sensitizing step may vary depending on the particular alloy. Also, lower sensitizing temperatures generally require longer sensitizing time periods, whereas higher sensitizing temperatures generally require shorter sensitizing time periods to achieve comparable effects.

In come embodiments, the sensitizing step can be carried out as a plurality of sensitizing steps, such that a sensitizing step at a relatively low temperature may be used to form a fine distribution of grains in the aluminum alloy, followed by one or more subsequent sensitizing steps at successively higher temperatures which may be used to increase the speed of coarsening once precipitates have been formed in order to provide sufficiently coarse grains. In these embodiments, beginning the sensitizing step with a relatively lower temperature increases the driving force for grain formation, thereby increasing the number density of grains, and continuing the sensitizing process with a relatively higher temperature decreases the sensitizing time and may also enhance the economy of the process.

The alluminum alloy may be cooled after each sensitizing step, typically by air cooling, which is also easier and less-costly to implement than other cooling methods.

The aluminum alloys treated by the process of the present invention have superior corrosion resistance. The corrosion resistance of the treated aluminum alloy may be assessed by a known mass loss test in compliance with the ASTM G67 standard.

To prepare the alloys for the mass loss test, a desmutting step may be carried out to remove residual substances on the sample's surface. A sample may be desmutted by placing the alloy in a sodium hydroxide solution and then in nitric acid for sufficient time.

The grain structure of the treated aluminum alloys may be analyzed by using a scanning electron microscope for electron backscatter diffraction (EBSD). To achieve electron diffraction standard polishing, mount procedures may be used with a final 0.03 micron colloidal silica polishing.

The present invention produces aluminum alloys that have a substantially reduced susceptibility to corrosion, especially seawater corrosion, in some cases by at least a factor of 2. The aluminum alloy treated by the process of the present invention is particularly suitable for naval vessels where seawater corrosion is of a significant concern. Thus, use of this process on an industrial scale to produce highly corrosion resistant aluminum alloy for naval vessels could reduce maintenance on naval vessels due to seawater corrosion.

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

EXAMPLES

The following examples are illustrative, but not limiting, of the methods and compositions of the present disclosure. Other suitable modifications and adaptations of the variety of conditions and parameters normally encountered in the field are within the scope of the disclosure.

Example 1

Aluminum alloy AA5083-H116 was treated using the process according to the present invention. A control sample was not treated, but cut to size and sensitized for comparison. Another control sample was cut to size but not sensitized or treated.

Each sample was cut and ground to a thickness larger than their respective reduction ratio to assure equal final thicknesses of all samples (processed samples and control samples). The treated samples were first annealed at 400° C. for a period of 1 hour. After annealing, these samples were cold rolled to achieve reductions of 0%, 10%, 20%, 30%, and 50% of the original thickness of the samples, leaving all samples with a final thickness of ˜6 mm. Each sample's width and length were adjusted by grinding to be 6 mm by 50 mm. Half of the samples were heat treated at 500° C. and the other half at 400° C., using an equal number of samples from each cold rolling thickness reduction category. After heat treatment, all samples (except the control sample that was not sensitized) were sensitized for 100 hours at 150° C.

To prepare all of the samples for the mass loss test, a desmutting process was carried out to remove residual substances on the samples' surfaces. Samples were placed in a sodium hydroxide solution and then in nitric acid for sufficient time to clean the surfaces. The samples were sent to be tested for 24 hour mass loss in temperature controlled nitric acid at Carderock's Naval Surface Warfare Center. All the mass loss test procedures were completed in compliance with the ASTM G67 standard.

The results of the ASTM G67 mass loss test for the treated and control AA5083 samples show that the sample with the least mass loss were those that had been cold rolled to a 10% thickness reduction and heat treated at 500° C. (FIG. 2). The samples heat treated at 500° C. showed the lowest corrosion loss at lower cold rolling thickness reduction percentages but corrosion loss increased at a 20% cold rolled thickness reduction and above. At cold rolling thickness reductions greater than 20%, the samples that were heat treated at 500° C. tended to lose more mass in most cases than the samples that were heat treated at 400° C. This trend was confirmed by the data of FIG. 3.

Example 2

In this example, the same sample treatment procedure as Example 1 was carried out, except that the cold rolling thickness reduction percentages were 5% and 15%. The results were consistent with the observations from Example 1 as shown in FIG. 3. The samples cold rolled to a 5% thickness reduction and heat treated at 500° C. outperformed all other samples showing a mass loss per area of less than 10 mg/cm².

EBSD scan images of the samples captured the grain structures within the best and worst performing samples in terms of mass loss in Example 2. The grain structure of the sensitized control sample (FIG. 4A) showed small grain sizes as compared to the relatively large grains of the sample treated with a 5% cold rolling thickness reduction and a 500° C. heat treatment (FIG. 4B).

It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meanings of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A process for grain boundary engineering of an aluminum alloy of AA5XXX series, said process comprising steps of: annealing said aluminum alloy at a temperature of from about 350° C. to about 450° C.; deforming said annealed aluminum alloy to reduce the thickness by from about 2% to about 20% of the original thickness of the aluminum alloy; and heat treating said deformed aluminum alloy at a temperature of from about 450° C. to about 550° C.
 2. The process of claim 1, wherein said temperature in said annealing step is from about 370° C. to about 430° C., or from about 380° C. to about 420° C.
 3. The process of claim 1, wherein said annealing step is performed for a period from about 0.5 to about 3 hours, or from about 1 to about 2 hours.
 4. The process of claim 1, further comprising the step of cooling the annealed aluminum alloy before the deforming step.
 5. The process of claim 4, wherein said cooling step has a cooling rate of from about 10 to about 30° C. per minute, or from about 15 to about 25° C. per minute, or from about 18 to about 22° C. per minute.
 6. The process of claim 1, wherein said deforming step comprises cold rolling.
 7. The process of claim 6, wherein said cold rolling reduces the thickness from 3% to 15%.
 8. The process of claim 6, wherein said cold rolling reduces the thickness from 5% to 10%.
 9. The process of claim 6, wherein said cold rolling is performed at room temperature.
 10. The process of claim 1, wherein said temperature in said heat treating step is from 475° C. to 525° C.
 11. The process of claim 1, wherein said temperature in said heat treating step is from 485° C. to 515° C.
 12. The process of claim 1, wherein said heat treating step is performed for a period of from about 1 to about 6 hours, or from about 2 to about 5 hours, or from about 2 to about 4 hours.
 13. The process of claim 1, further comprising the step of sensitizing said heat treated aluminum alloy.
 14. The process of claim 13, wherein said sensitizing step is performed at a temperature of from about 100° C. to about 200° C., or from about 120° C. to about 180° C., or from about 140° C. to about 160° C.
 15. The process of claim 13, wherein said sensitizing step is performed for a period of from about 70 to about 150 hours, or from about 80 to about 130 hours, or from about 90 to about 120 hours.
 16. The process of claim 13, wherein said sensitizing step comprising a plurality of sensitizing steps.
 17. The process of claim 16, wherein each of said plurality of sensitizing steps is performed at a different temperature.
 18. The process of claim 16, wherein each of said plurality of sensitizing steps is performed with a low temperature sensitizing step followed by sensitizing steps at higher temperatures.
 19. The process of claim 16, wherein each of said plurality of sensitizing steps is performed using a cooling step following each sensitizing step.
 20. An aluminum alloy of AA5XXX series treated by the process of claim
 1. 